This invention relates to melt-stable, titanium dioxide-containing copolymers of ethylene with alpha,beta-unsaturated carboxylic acids, which are useful, i.a., in laminated structures such as, e.g., toothpaste tubes, where metal foil is laminated to a printable plastic layer on which the name of the product or other similar information is to be printed. Ethylene copolymers with alpha,beta-unsaturated carboxylic acids are known to be very good hot melt adhesives, which are suitable for bonding the outer layer to the metal foil or to another plastic layer. When the outer layer is a clear or at least translucent plastic, it is considered advantageous from the standpoint of aesthetics and commercial appeal that the adhesive layer be white-pigmented, so that the white color may show through. Another typical application for such white-pigments copolymers is in condiment pouches.
Traditionally, titanium dioxide has been used as white pigment for plastics and for other purposes. However, commercial titanium dioxide particles normally are coated on their surface with alumina, which improves the pigment's flowability. When such alumina-coated titanium dioxide is incorporated into an ethylene/alpha,beta-unsaturated carboxylic acid copolymer, and such pigmented copolymer is exposed to high temperatures, such, for example, as are encountered during extrusion, the copolymer appears to be reacting with the alumina coating, as evidenced by a decrease of the polymer's melt index. Further reduction of melt index occurs on storage. Such lowering of the melt index is undesirable because it leads to nonuniform coating thicknesses and pinholes in the coatings. In addition to the instability of the melt index, a further problem has been the formation of gels in the polymer, resulting in poor quality extrusion coatings.
There is, therefore, a great need for a thermally stable titanium dioxide-pigmented ethylene/alpha,beta-unsaturated carboxylic acid copolymer.